Voltage Controlled Crystal Oscillator Terms and Definitions
A VCXO is a quartz crystal oscillator that includes a varactor diode and associated circuitry allowing the frequency to be changed by application of a voltage across that diode. By varying the control voltage, the capacitance of the varactor changes accordingly, thus forces the output frequency changed. This can be accomplished in a simple logic clock or sine wave crystal oscillator. In addition to those terms that define the fixed frequency crystal oscillator, there are several characteristics peculiar to VCXOs.
Control voltage (Vc):
An external voltage applied to the input of the VCXO. By changing the voltage, the frequency varies accordingly. Typical Vc is 0V to5V; 0.5V to 4.5V; -0.5V to 4.5V.
Deviation or Frequency pullability:
The minimum change in the output frequency with respect to the change in control voltage. Unit is measured in ppm. Standard pullability is ±50ppm minimum, ±100ppm minimum.
The deviation from the best straight-line slope of the frequency vs. control voltage curve. If the specification for an oscillator requires a linearity of ±10% and the actual deviation is 10 kHz total as an example, the curve of output frequency vs control voltage input could vary as much as ±1 kHz (10 kHz ±10%) from the Best Straight Line. On the other hand, if maximum deviation from the Best Straight Line is 20 ppm and the Total deviation is 200 ppm, the linearity is ±20 ppm/200 ppm = ±10%, which is the linearity value for a typical VCXO.
A measure of isolation between the input port of the VCXO network and the voltage control source. Typical input impedance is > 50 Ohms @ 10 Khz.
Center frequency or nominal frequency:
User specified frequency at center control voltage. Standard Control voltage for center frequency is 2.5V for 5V applications (with control voltage range 2.5V ±2.0V) and 1.65V for 3.3V applications (with control voltage range 1.65V ±1.5V)